This invention relates to the method of measuring and characterizing the spatial distribution of color across a light spot created from a lighting apparatus, and in particular to the measurement and characterization of light spots with high color uniformity, created from Light Emitting Diode sources and subsequent light shaping and mixing optics.
Colored (non-monochromatic) lights have been used in numerous applications such as cinematography, photography, and architectural lighting (both commercial/industrial and residential). A high quality light spot created from a light source is characterized by its spatially uniform chromaticity, or color temperature when characterizing white light. Achieving a spatially uniform light spot has not been challenging to do with traditional light sources such as tungsten bulbs and HMI (hydrargyrum medium-arc iodide) lamps since it is easier to produce a uniform light spot from a uniform source rather than a non-uniform one. However, producing a uniform light spot is more of a challenge for some modern light sources that rely on the mixing of light from multiple discrete light sources such as LEDs (Light Emitting Diodes), which typically require additional light shaping and/or mixing optics such as diffusers to achieve good color uniformity. Although there are LED based light sources which are similar to traditional light sources in that the individual LED chips all emit the same color and hence make up a uniform source, many LED light sources are composed of multiple discrete LEDs, or an array of LED chips, of two or more different colors. As example, in FIG. 1 an LED array light source (101) is composed of chips of two colors (102, 103) and without spatial light mixing optics the non-uniform colored light source produces a non-uniform colored light spot (104).
Because color uniformity across the light spot is so important, there is a need to measure it both in research and development, as well as in production and in product use. Light measurement integrating spheres can be used to measure the chromaticity of the total light coming from an LED array but they do not indicate or measure the spatial distribution of the chromaticity of the light. Furthermore, although an integrating sphere can measure the intensities of different colors, it does not measure the sharpness or gradient of the transition between colors across the light spot. For instance, a spot having a blue region that gradually transitions to a red region looks different from one where the transition is sharp or abrupt, creating an easily visible edge.
The desired detailed spatial color distribution can be measured by smaller light detectors such as colorimeters which can measure chromaticity at different regions as the detector is physically moved to different locations on the light spot. The primary disadvantage of using a traditional colorimeter and moving it to different positions on a light spot is that the measurements must be done serially, one after another, and hence take significant time if good spatial resolution is to be achieved. Furthermore, the size of commercially available colorimeters or detectors is typically on the order of a square inch which may be comparable to, or of the same order of magnitude as, the size of the light spot being measured, thereby limiting the maximum spatial resolution.
It is therefore desirable to provide an improved method of measuring and characterizing the spatial distribution of color across a light spot created from non-uniform light sources, such as Light Emitting Diodes.